Fiber-reinforced composites manufactured by reinforcing plastic materials (called matrix resins) with various synthetic fibers are employed widely in automotive industry, aviation and space industry, sporting and leisure goods industry, and general industries. Fibers employed for the composites include inorganic fibers, such as carbon fiber, glass fiber and ceramic fiber, and organic fibers, such as aramid fiber, polyamide fiber and polyethylene fiber. These synthetic fibers are usually produced into continuous filament, and later processed into various forms of reinforcement textiles with advanced techniques, including a sheeted intermediate material called unidirectional prepreg which is manufactured by applying hot melt resin to fabric and winding it onto a drum, textile material manufactured by filament winding, and sometimes woven fabric or chopped fiber.
Reinforcement fibers are often used in a form of chopped fiber cut into 1 to 15 mm long for manufacturing fiber-reinforced composites of thermoplastic resins, such as polyolefin resins, polyamide resins, polycarbonate resins, polyacetal resins, ABS resins, polyphenylene sulfide resins and polyetherimide resins, which are included in the matrix resins mentioned above and attract attention because of their good moldability and advantages in recycling. The chopped fiber should have sufficient cohesion when it is knead with a thermoplastic resin to be manufactured into pellets, because chopped fiber having insufficient cohesion may be fed inconstantly to pellet manufacturing or break to deteriorate the properties of resultant fiber-reinforced composites. For preventing such troubles, numbers of techniques for coating reinforcement fibers with sizing agents formulated of various thermoplastic resins as a main component have been proposed in order to impart optimum cohesion to reinforcement fibers (for example, see Japanese patent documents JP-A-06-002274, JP-A-2002-138370 and JP-A-2003-165849 and widely employed in industrial fields.
Recently, reinforcement fibers are increasingly processed into a form called filament pellet or into a unidirectional sheet, tape or fabric to be impregnated with thermoplastic resins and molded in subsequent processes similarly to composite materials of thermosetting resins, in order to effectively achieve desirable properties of reinforcement fibers including tensile strength. In such cases, hot-melt thermoplastic resins should rapidly penetrate into fiber strands, specifically, fill space between single fibers when a fiber-reinforced composite is molded, in order to shorten molding time and improve the physical properties of resultant composites.
Sizing agents in prior art have posed problems relating to insufficient performances of sizing agents including low bonding strength between a matrix resin and sized reinforcement fiber due to insufficient heat resistance of the sizing agent or poorly impregnated reinforcement fiber with a matrix resin.
More serious problem relating to conventional sizing agents is low bonding strength between a matrix resin and sized reinforcement fiber caused by nonuniform coating on the reinforcement fiber with a sizing agent, in other words, slow penetration of a sizing agent into fiber strand leading to nonuniform spreading of the sizing agent between single fibers or on fiber surface. In addition, conventional sizing agents form a strong film on fiber surface to make sized fiber strands poorly splittable, and thus disturb quick splitting of sized reinforcement fiber strands at guide bars in warping before sized reinforcement fiber strands are processed into unidirectional sheet or woven fabric. Such sizing agents have also caused other problems including stiffly sized reinforcement fiber strands which cannot be easily wound into packages, or the deformation of packages of sized reinforcement fiber strands during transportation.